As capacity and power of lithium ion rechargeable batteries have increased in recent years, the demand for improvement in mechanical strength such as a pin puncture strength and withstand voltage characteristics such as a dielectric breakdown voltage has been growing from the viewpoint of safety. In addition, thinner separators are being developed for lithium ion rechargeable batteries because the shorter distance between battery electrodes is preferable for a larger capacity and a higher output of the lithium ion rechargeable batteries. Therefore, the demands for improved mechanical strength and withstand voltage characteristics in lithium ion rechargeable batteries have increased from the viewpoint of preventing membrane puncture and shorting.
Patent Document 1 discloses a technology for improving withstand voltage characteristics and pin puncture strength by blending inorganic particles of a certain size to polyolefin. However, blending a large amount of fine particles in polyolefin impairs uniform dispersion of the fine particles and, because these particles are removed after the membrane formation, the membrane porosity increased excessively, resulting in insufficient withstand voltage characteristics. In addition, because the microporous membrane was formed by a process that requires a large amount of fine particles, there were issues regarding the process, the equipment, and maintenance.
Patent Document 2 discloses a technology for improving shut-down characteristics by blending a nucleating agent to a layer including polyethylene having a high MFR value to form a multilayer microporous membrane having a fine structure. However, there is a problem of decrease in the pin puncture strength if the polyethylene having an MFR value in this range, compared to the case where the high molecular weight polyethylene is used.